There is considerable interest in monitoring properties of semiconductors at various stages during the fabrication process. Monitoring the properties during fabrication allows the manufacturer to spot and correct process problems prior to the completion of the wafer.
Inspection of actual product wafers during or between process steps usually require non-contact techniques. Accordingly, a number of tools have been developed for optically inspecting semiconductor wafers. Such tools include reflectometers and ellipsometers. To increase the robustness of the measurements, these tools can often obtain measurements at multiple wavelengths and/or multiple angles of incidence.
Therma-Wave, Inc., the assignee of the subject invention has developed a number of such tools over the last fifteen years. One of the first such tools is described in U.S. Pat. No. 4,999,014. In this tool, a probe beam from a laser is tightly focused on the sample with a high numerical aperture lens to create light rays with a spread of angles of incidence. The reflected beam is imaged onto an array detector. The location of the elements on the array detector can be mapped to different angles of incidence on the sample. Thus, this device can obtain reflectometry information at multiple angles of incidence with no moving parts. This configuration is still in commercial use today in Therma-Wave's Opti-Probe product line and is referred to as Beam Profile Reflectometry® or BPR®.
This concept was subsequently extended to ellipsometric measurement as described in U.S. Pat. No. 5,042,951. In this approach, the change in polarization state of the probe beam is monitored at multiple angles of incidence. Various polarizers and a waveplate (or compensator) are used to permit the polarization analysis. A variant of this approach which integrates the angular information is disclosed in U.S. Pat. No. 5,181,080. This configuration is also in commercial use and is referred to as Beam Profile Ellipsometry® or BPE®.
While the latter patents were directed primarily to single wavelength systems, efforts have been made to extend these concepts to multiple wavelength systems. See for example, U.S. Pat. Nos. 5,412,473 and 5,596,411.
The assignee herein has also made efforts to improve broadband spectroscopic ellipsometry. More specifically, and as described in U.S. Pat. No. 5,877,859, an improved spectroscopic ellipsometer was proposed that utilized a rotating compensator (waveplate). Prior to this disclosure, rotating compensators were typically used only in narrow band ellipsometers, while rotating polarizers were used in broadband, spectroscopic ellipsometers. U.S. Pat. No. 5,877,859 discloses how a rotating compensator can be used in BPR and BPE type systems.
The photodetectors used in the basic BPR and BPE systems usually were defined by two linear arrays of photodetector elements oriented orthogonally to each other. Some of the above cited patents suggest that a two dimensional CCD array could be used. In some cases, the CCD array was used to measure both wavelength and angle of incidence information. The use of a CCD array in conjunction with various types of BPE measurements was proposed in U.S. Pat. No. 6,678,046 and in U.S. patent application No. 2005/0041258.
Each of the above cited patents and patent application are incorporated herein by reference. Also relevant are U.S. Pat. Nos. 6,278,519; 6,429,943; 5,166,752, 5,889,593; 5,910,842; 6,268,916; 6,483,580; 2001/0051856; 2004/0004726 and 2004/0070772 each of which is incorporated herein by reference.